The invention relates generally to the communication of broadband signals and, more particularly, to the processing of signals carried in dissimilar media.
Broadband communication systems in use today often utilize dissimilar media throughout the signal path of transmitted signals. For example, cable television infrastructure often utilizes fiber optic media for long haul transmission of broadband signals and coaxial cable or other electrically conductive media for signal processing, head-end connections, and/or subscriber premise connections. Accordingly, transmission of broadband signals including multiplexed television channel signals, data signals, such as Internet content data, pay per view data, and other data, may traverse various media including fiber optic cable, coaxial cable and other media.
Analog optical receivers have been used in the prior art to bridge an optical media and electrically conductive media such as an optical fiber and cable interface. An analog optical receiver typically takes a current generated from an optical PIN diode interfaced with a fiber optic signal path and creates a corresponding voltage for signal processing using electrically conductive media, e.g., input into a tuner or transmission via coaxial cable. However, the currents generated from optical PIN diodes are very small and the voltage output typically required for desired signal processing is very large comparatively. Moreover, the relationship of the output voltages to the input current must be highly linear with very low distortion. Accordingly, analog optical receivers used with broadband signals must be very low noise and very linear, as well as providing a large frequency response or bandwidth.
Analog optical receivers in existence today are discrete solutions, typically having a fixed gain input amplifier, a variable attenuator, and multiple stages of fixed gain amplifiers and variable attenuators. For example, prior art analog optical receivers often utilize a fixed gain transimpedance amplifier (TIA) coupled to a PIN diode attenuator followed by a plurality of fixed gain amplifiers and attenuators, the number of which depending upon the range of attenuation provided by the PIN diode attenuator and system gain required. TIAs are relied upon in such circuits to convert low-level photodiode currents to usable voltage signals. Alternative embodiments implemented in the prior art utilize a winding or transformer, to provide current gain, coupled to a fixed gain voltage amplifier, to provide voltage gain, in place of the above described transimpedance amplifier. Accordingly, a fixed gain transimpedance stage is again provided which is coupled to a variable attenuator according to this alternative prior art embodiment.
The above prior art analog optical receiver architectures suffer from several undesired characteristics. For example, because the variable attenuator is disposed in the circuit to attenuate the signal after the input amplifier, the input amplifier must have a very linear response. Specifically, the amplifier""s output intercept point, a measure of the linearity of its output signal, is attenuated with a decrease in gain. Accordingly, the fixed gain amplifier must have an output intercept that is much greater than is actually required to provide proper signal quality by the maximum amount of attenuation to be utilized. That is, assuming a design criteria of a third order output intercept (OIP3) equal to 0 dBm in an analog optical receiver with a PIN diode attenuator providing variable attenuation up to 20 dB, the input amplifier must provide an OIP3 of 20 dBm (0 dBm as required by system parameters plus 20 dBm attenuated worst case by the variable attenuator). In the above example, the input amplifier""s input intercept, which is equal to the device""s output intercept minus its gain, would remain constant.
As the output intercept of a circuit component is a measure of linearity, the prior art analog optical receivers require input amplifiers which are considerably more linear than design parameters otherwise require. Typically such highly linear amplifiers are more costly and difficult to implement and result in higher power consumption circuit designs than is desired. Moreover, the output intercept is effectively variable with gain, i.e., the input amplifier and variable attenuator circuit output intercept increases with signal gain.
A need therefor exists in the art for analog optical receivers which provide desired signal processing attributes and which may be economically implemented. A further need exists in the art for such analog optical receivers to implement architectures which are power efficient.
A still further need exists in the art for a transimpedance amplifier configuration adapted to provide amplification of a current source signal to voltage levels suitable for further signal processing without an associated variation in output intercept.
The present invention is directed to systems and methods which provide signal processing circuitry utilizing an amplifier configuration which is economical to implement and which provides for power efficient circuit implementation. Preferred embodiments of the present invention provide an amplifier having an output intercept that remains constant while a corresponding input intercept increases as gain is reduced. A most preferred embodiment of the present invention provides a constant output intercept, variable gain, transimpedance amplifier.
For example, an analog optical receiver of the present invention preferably implements an architecture in which a variable gain transimpedance amplifier is disposed as an input amplifier. According to this preferred embodiment analog optical receiver, the linearity requirements of circuitry, such as the amplifier circuitry itself and a variable attenuator coupled to the amplifier circuitry, are lessened, thereby providing advantages such as ease of implementation, ability to implement the circuits as integrated circuits, and reduced manufacturing costs.
Moreover, as the role of a typical optical receiver is to present a constant output voltage or power under varying input optical power conditions by varying the gain of the optical receiver, the preferred embodiment configuration is particularly useful in analog optical receiver implementations because the output intercept remains constant and the input intercept increases as signal gain is decreased. Specifically, as the input power to the optical receiver increases, the gain must be reduced, and therefore, it is desirable that the input intercept, which relates to the maximum input signal for a given linearity, increases with decreasing gain to cope with the increasing input power.
Preferred embodiments of the present invention utilize a high gain amplifier effectively implementing a variable resistor feedback network. For example, a variable resistor feedback network may be provided according to the present invention through a piece-wise linear resistor function. Variable resistor feedback may be implemented by switching in various values of resistance through use of buffer stages, if desired.
Transimpedance amplifiers of the present invention are preferably readily implemented in an integrated circuit, such as upon a silicon substrate. For example, linearity requirements of particular circuits, such as the aforementioned analog optical receiver circuits, are attainable implementing a variable gain transimpedance amplifier of the present invention which is less complicated, and therefore simpler to construct monolithically, than a typical amplifier implemented according to the prior art. Moreover, as preferred embodiment analog optical receivers of the present invention implement an architecture in which a highly linear variable attenuator, such as the aforementioned PIN diode attenuator, is not required, preferred embodiments of the present invention provide for complete integrated circuit implementation of circuits associated therewith. For example, an analog optical receiver of the present invention may be fully integrated on a single silicon substrate. In contrast, in addition to input amplifiers providing sufficient linearity for prior art analog optical receiver architectures being difficult to implement in an integrated circuit, the PIN diode attenuators required by such architectures are typically impractical to implement in an integrated circuit and, therefore are implemented in discrete components.
A technical advantage of the present invention is that analog optical receivers configured to provide desired signal processing attributes may be economically implemented. A further technical advantage of the present invention is that preferred embodiment analog optical receivers implemented according to the present invention are power efficient.
A still further technical advantage of the present invention is that a preferred embodiment transimpedance amplifier configuration provides amplification of a current source signal to voltage levels suitable for further signal processing without an associated variation in output intercept.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.